The present invention relates to the measurement of characteristics of a video signal, and more particularly to K2T measurement of video signals extended to formats other than standard definition television (SDTV).
The problem addressed is how to extrapolate a subjectively correlated standard method of objectively measuring video quality degradation due to linear distortions for standard definition television (SDTV) to other video formats, such as high definition television (HDTV) and computer video for example. In other words there is a need to predict subjective quality at these other video formats.
Linear distortions are important in analog video testing because they can directly and indirectly cause visible degradation. A virtually infinite combination of the linear distortions, such as amplitude and delay versus frequency, can have a similar impact on the perceived quality in video. For objective measurements of the impact of linear distortions on video quality, the approach has been to use a test pulse with a limit template to calculate the gain ratio (graticule to test signal) required to contain the video signal within the graticule. A K Factor rating system is used to map linear distortions of 2T sine-squared pulses, that are themselves bandwidth limited, onto subjectively determined scales of picture quality. The sine-squared pulses are specified in terms of half amplitude duration (HAD), which is the pulse width measured at 50% of the pulse amplitude. T is the Nyquist interval, or 1/fc where fc is the cutoff frequency of the system to be measured. K2T is a weighted function of the amplitude and time of the distortions occurring before and after the 2T pulse. This ratio in percent correlates well to subjective quality ratings of video impaired by linear distortions. See C. A. Siocos and G. Chouinard, “SUBJECTIVE IMPAIRMENT UNITS IN RELATION WITH OSCILLOSCOPE GRATICULES FOR EVALUATING SHORT-TIME LINEAR WAVEFORM DISTORTIONS,” IEEE Transactions on Broadcasting, Vol. BC-25, No. 2, Jun. 1979.
As mentioned above, the prior approach has been to use a test pulse, such as that shown in FIG. 1 where the half amplitude duration (HAD) is 2 times T, with a limit template as shown in FIG. 2 to calculate the gain ratio, either graticule or test signal, required to contain the video signal within the graticule. FIG. 2 shows an example of a K2T graticule according to CCIR Recommendation 451 vertically normalized to 2T pulse amplitude and horizontally normalized to T. The right of pulse center is shown, but the limits apply equally to both the left and right side of pulse center. This ratio multiplied by a K factor of the nominal graticule, such as 0.04 specified in CCIR Recommendation 451, in percent (K2T %) becomes the measurement result. Graticules and associated automated measurements meeting this specification have been implemented in analog, digital and software forms. The Tektronix® VM700 and VM100 video measurement products both include implementations of this graticule for standard definition K2T measurements.
However standards do not yet exist for graticules in other video formats, such as high definition and computer video. The standard definition (SD) graticule specifications are specified for a much lower bandwidth than the high definition (HD) and computer video formats require. Simply using bandwidth ratios of new and old formats to scale graticules of the existing standard does not give the intended result. The test pulse is designed to contain spectral content approximately up to the bandwidth limit, approximately 1/T, of the format. Simply scaling the existing standard graticule using the new T value results in a measurement that does not track visual sensitivity to impairments, as shown in FIG. 3 where both the old CCIR graticule is shown scaled for HD and the new graticule for HD calculated according to the present invention, as explained below, is shown. For example the high definition 2T30 pulse is so narrow that under standard viewing conditions the eye is much less sensitive to impairments than the same time normalized impairments. And finally, re-scaling according to line time, as described in U.S. Patent Publication No. 2003-0160515-A1, is not sufficient because the re-scaled graticule does not correspond to the difference in test signal spectrum—see FIG. 4. The spectrum (normalized approximate luminance in dB relative to max vs. spatial frequency or sequency in units of cycles per visual degree) of 2T4 MHz Pulse (SD) vs. 2T30 MHz Pulse (HD) shows the comparison of visual stimulus of SD vs. HD 2T pulses under comparable viewing conditions. Since human vision sensitivity is a function of spatial frequency, the associated graticule for HD needs to be modified in a non-uniform manner rather than simply being time scaled.
What is required is a perceptually consistent K2T measurement algorithm to summarize the video quality impact of linear distortions present in analog video and equivalent digital representations across multiple formats, and in particular to have one method of measurement for standard definition as well as other formats including high definition and computer video with various T, sample rate, samples per line and aspect ratio parameters.